1. Field of the Invention
The present invention relates to power supply arrangements for multi-stage amplifiers. The invention is particularly but not exclusively concerned with power supply arrangements for multi-stage amplifiers which utilise envelope tracking techniques.
2. Description of the Related Art
In a multi-stage amplifier arrangement, a plurality of amplifier stages are provided in series for amplification of a signal. An example multi-stage amplifier arrangement for amplification of a radio frequency (RF) signal may comprise a driver amplifier stage followed by a power amplifier stage. Such a two-stage amplifier arrangement is typically provided in a transmitter in a handset or mobile device of a mobile communications system.
In order to achieve efficiency in the operation of power amplifier arrangements, various schemes have been disclosed in the prior art for efficiently providing a supply voltage to an amplifier stage, and particularly to a power amplifier stage. A particularly advantageous high efficiency amplification scheme is disclosed in European Patent No. 1597821 in the name of Nujira Limited.
In multi-stage amplifier arrangements, it is known in the prior art to apply a modulated supply voltage simultaneously to two or more amplification stages. Such an approach is successful for the amplification of narrowband signals, such as GSM/EDGE 200 kHz bandwidth signals.
However the envelope tracking principles of a high efficiency tracking supply place tight limits on time alignment between the various signal paths in the system. For narrowband signals, these limitations can be handled. However for wideband signals, such as in wideband code division multiple access (WCDMA) systems, world interoperability for microwave access (WiMAX) systems, or long term evolution (LTE) systems for example, these requirements create difficulties in providing supply modulation on more than one amplifier stage at any given time.
In the prior art, therefore, for a multi-stage amplifier arrangement where a highly efficient modulated power supply stage is provided using envelope tracking, typically such supply modulation is provided for only one amplifier stage. The one or more remaining amplifier stages are arranged to operate from fixed supply voltages. The operation of the one or more remaining amplifier stages based on fixed supply voltages results in poor operating efficiency. This poor efficiency is particularly relevant for systems where the RF output can vary over a large dynamic range, and/or the RF signal has a high peak-to-average power ratio (PAPR).
It is an object of the invention to provide a multi-stage amplifier arrangement in which efficiency of performance is improved.